Mixing Plaster of Paris for Mold Making

WEIGHING

Weigh both the plaster
and water for each mix. The water-to-plaster ratio is critical because it
governs both the strength and the absorptive capacity of the mold. Use
Consistency is a measure of this ratio and represents parts of water by weight per 100 parts plaster. You can vary the consistency successfully between 74 to 82 with the lower numbers creating higher strength molds and the higher numbers creating higher absorption.

WATER

Use potable water at temperatures between 70° and 100°F. Variations in temperature
produce variations in setting time. The higher the
temperature of the water, the shorter the set time.

SIFTING

Sift the plaster by hand into water slowly and evenly. Do not drop handfuls of
plaster directly into the water. Allow soaking for 1-2 minutes. The plaster
should be fully dispersed in the water before mixing. Small batches require
less soaking than large batches.

POURING

Carefully pour the mixture into the case to
prevent air entrainment and provide a uniform, smooth surface. Bump or agitate the case during pouring and right after pouring to prevent air bubbles at or near the surface. When
possible pour the plaster into the deepest area
so the slurry flows evenly across the surface of the case. Don't pour a large amount of slurry directly on one spot
of the case mold or you may get densification of the plaster
at the point where it strikes the surface of the case. This produces a hard
spot, giving uneven absorption.

DRYING

Dry quickly to a constant weight. Provide uniform and
rapid circulation, equal temperatures throughout the
area, and provisions for exhausting a portion of the air while
replacing it with fresh air. The maximum temperature at which plaster molds are safe
from calcination is 120°F. With substantial free water in
the mold, higher drying temperatures can be used without difficulty. As
drying progresses, the temperature must be reduced to prevent calcination.
Before removing molds from a dryer, the temperature should approach that of the
area around the dryer to prevent thermal shock.

STORAGE

Keep molds in a dry, stable environment indoors. Do not stack more than 2 pallets high.
Keep from drafts. Rotate stock.